1. Field of the Invention
The present invention relates to the storage of well fluids produced from offshore wells. More specifically, the present invention relates to a submergible storage system for the temporary storage of oil produced from wells completed in deep, remote offshore locations.
2. Description of the Prior Art
Oil and gas produced from offshore wells must be transported from the site of the well to a land-based, refining, storage, or transportation facility for subsequent processing and dispersement to the end users. In most situations, such production fluids are transported either via a submerged pipeline or by a tanker from temporary storage facilities located at the well site. In some cases, a large production facility may process the well fluids before they are transported by the tanker or pipeline to the final user.
Typically, the economies of building and operating an offshore production facility require that a large number of wells, or a few very prolific wells, be in the near vicinity of the facility and connect to the facility by submerged pipeline. Similarly, the cost of laying a pipeline to an offshore production area is justified only if there is a relatively large number of wells in the offshore area and they are not too distant from an onshore facility.
In many cases, where the amount of oil produced is limited, the cost of providing a pipeline or an intermediate production facility for the temporary storage of oil produced from a single well or an isolated field cannot be justified. In such situations, the use of tankers to collect oil from temporary storage facilities at the remote well or field may be the only practical way to secure the production from these isolated or minimally producing wells.
The temporary storage of oil and other fluids produced from some wells has, in the past, been accomplished by constructing a production platform with storage facilities at the site of the well and off-loading the stored fluid periodically into a tanker. Additionally, in some cases, underwater storage tanks or facilities have been employed to store production from these wells. In either case, these prior art methods for temporary storage of the well fluids have been employed in relatively shallow water, where the cost of erecting a platform or placing an underwater storage tank is reasonably in proportion to the value of the hydrocarbon product being produced at the remote site. As the water depth increases, the cost of production platforms or submerged storage facilities increases accordingly and, in deeper water, use of such systems has not proven economical.
In some areas, even in relatively shallow water, and even where production gathering facilities are close enough to be connected by a relatively short submerged pipeline, the cost of building a conventional production platform cannot be justified based on the amount of production anticipated from the well. Accordingly, there is a class of wells that could be economically drilled and produced if the cost of fabricating a suitable production platform were economical. In such cases, the addition to the site of adequate temporary surface storage or, even submerged temporary storage, can make production from the site economically desirable.
Oil and gas have been economically produced from marginally producing wells located in relatively shallow, accessible locations where the production platform can be of a caisson-supported design, which is significantly less costly than a typical production facility. An example of an economical, caisson-mounted production system is described in U.S. patent application Ser. No. 08/573,594, assigned to the assignee of the present invention. In the described caisson production completion system, the production platform is supported by a single caisson that eliminates the need for a multi-leg platform that is anchored into the water bottom. The savings realized from completing the well with a caisson system as described in the patent application can make a marginal well economically profitable.